Retire "Idle Threads Sleep" UCI option

After last Joona's patch there is no measurable
difference between the option set or unset.

Tested by Andreas Strangmüller with 16 threads
on his Dual Opteron 6376.

After 5000 games at 15+0.05 the result is:

1 Stockfish_14050822_T16_on   : 3003  5000 (+849,=3396,-755), 50.9 %
2 Stockfish_14050822_T16_off  : 2997  5000 (+755,=3396,-849), 49.1 %

bench: 880215

src/thread.h

@@ -166,7 +166,6 @@ struct ThreadPool : public std::vector<Thread*> {
   void wait_for_think_finished();
   void start_thinking(const Position&, const Search::LimitsType&, Search::StateStackPtr&);
 
-  bool sleepWhileIdle;
   Depth minimumSplitDepth;
   Mutex mutex;
   ConditionVariable sleepCondition;